A Blu-ray Disc is an example of an optical information recording medium. An example of the layer structure of this medium is one in which a reflective layer, a third interface layer, a second dielectric layer, a second interface layer, a recording layer, a first interface layer, a first dielectric layer, and a cover layer are formed in that order on the surface of a substrate.
The function of the first dielectric layer and second dielectric layer is to adjust the optical distance (equal to the refractive index×the physical distance) and thereby raise the optical absorption efficiency of the recording layer, and increase the difference in reflectivity between the crystal phase and amorphous phase to increase signal amplitude. They also function to protect the recording layer from moisture and so forth. An example of the material of these dielectric layers is a mixture of 80 mol % ZnS and 20 mol % SiO2 (hereinafter referred to as (ZnS)80(SiO2)20) (see Japanese Patent Publication H-06-090808, for example). This is an amorphous material, which is characterized by low thermal conductivity, a high refractive index, and high transparency. It also can be formed into a film at a high rate, and has excellent mechanical properties and moisture resistance. Because of its outstanding characteristics, (ZnS)80(SiO2)20 has seen practical use as a material that is extremely well suited to forming dielectric layers.
The first and second interface layers are provided for the purpose of preventing the sulfur (S) in the (ZnS)80(SiO2)20 from diffusing into the recording layer when the recording layer is irradiated with laser light and repeated rewrite recording is performed. If the sulfur (S) diffuses into the recording layer, it markedly lowers the reflectivity of the medium, and the repeated rewrite characteristics of the recording medium are greatly compromised. A material containing ZrO2 and Cr2O3, for example, has been disclosed as the material for these interface layers (see Japanese Laid-Open Patent Application 2003-323743, for example). This material, containing no sulfur (S), is very transparent to a laser whose wavelength is in the blue-violet color band (near 405 nm), and has a high melting point, so the material also has high heat resistance.
The functions of the reflective layer are, optically, to increase the amount of light absorbed by the recording layer, and thermally, to quickly diffuse the heat produced in the recording layer to quench the recording layer, and facilitate a change to an amorphous state. Another function is to protect the recording layer, interface layers, and dielectric layers from the environment in which they are used. Therefore, a silver alloy, having high thermal conductivity, can be used favorably as the material for a reflective layer.
The function of the third interface layer is to prevent the sulfur (S) in the (ZnS)80(SiO2)20 from diffusing into the reflective layer when (ZnS)80(SiO2)20 is used for the second dielectric layer and a silver alloy is used for the reflective layer. If sulfur (S) does diffuse into the reflective layer, it will react with the silver in the silver alloy, producing Ag2S. This Ag2S is produced even in an environment of normal temperature and normal humidity, and markedly diminishes the reliability of the medium. The material of this third interface layer can be a dielectric other than a sulfide, or a metal other than silver, semimetal, or semiconductor.
The inventors discovered that a number of problems are caused by using a dielectric containing sulfur (S) for the second dielectric layer and providing both a second interface layer and a third interface layer.
One of the things the third interface layer does is to slow down the diffusion of heat produced in the recording layer. A good cooling effect in an information recording medium facilitates the change to an amorphous state, and results in good recording marks being obtained. Silver has the highest thermal conductivity of all elements, but as mentioned above, a silver alloy cannot be used for the third interface layer. Therefore, providing a third interface layer reduces the cooling effect on the recording layer. Also, if multiple layers of different materials are used for the interface layer, or if its thickness is increased, in an effort to improve how well elements are prevented from diffusing into one another, the cooling effect will suffer even more, quenching will be more difficult, and signal quality will decrease. Furthermore, providing a third interface layer increases the number of layers that make up the medium. Increasing the number of layers leads to an increased investment in equipment for manufacturing the medium, or results in a longer manufacturing takt time, which drives up the cost of the medium.
Similarly, providing the second interface layer decreases the cooling effect, and drives up the cost of the medium.